The centre-back is drafted in from Wolves, following the end of a half-season spell at Sheffield Wednesday, to help address an injury crisis plaguing the relegation-threatened side
The defender has just completed a spell at Sheffield Wednesday following the Molineux side's relegation to League One last season and now returns to Premier League football.
The Hammers are in the middle of an injury crisis, particularly in defence and put out a team largely comprised of youth for their 5-0 FA Cup hammering by Nottingham Forest.
Jack Sullivan, the son of co-chairman David Sullivan, confirmed the deal on Twitter, saying: "ROGER JOHNSON has signed till the end of the season, as emergency cover at CB. He might surprise a few people."
As noted in an open letter from club chairmen David Gold and David Sullivan, backing their manager, Sam Allardyce has only 14 fit first-team players at his disposal, with his side still in the Capital One Cup.
Johnson will be tasked with preventing his own fourth consecutive relegation, having dropped out of the Premier League with first Birmingham City and then Wolves in 2010-11 and 2011-12, before the latter suffered a double drop out of the Championship last season.